Body and Behaviours of sharks

They have five different kinds of fins, which are stiff. One or two dorsal fins, a pair of pectoral fins, a pair of pelvic fins and a caudal fin (the tail) work together to lift, propel and stabilise the shark. Some sharks have an anal fin.

The caudal fin, or tail, propels the shark through the water. It moves back and forth to propel the shark forward. Its upward movement points the shark's head downward.

The dorsal fin is the fin on the top of the shark's back, the one that shows above the water when the shark swims near the surface. It keeps the shark's body stable as it swims.

The second dorsal fin is on the top of the shark's body, between the big dorsal fin and the tail. It helps keep the shark stable as it swims.

Pectoral fins, one on each side of the shark's underside towards the front of the body, help lift the shark as it swims, and offsets the downward motion given by the caudal fins (tail), so that the shark moves horizontally through the water. •Many sharks have a small second pair of pectoral fins close to the tail to help keep the shark's body stable in the water.

An anal fin is a small fin on the underside of the body near the tail. Not all sharks have this.

A shark's Senses

Sharks have excellent eyesight, especially adapted for seeing in dim light under the sea. They can see moving objects from a great distance. Sharks also have excellent senses of hearing, smell and touch. In many kinds of shark the eyes are on the side of the head so they can see forward, backward, up and down.

Breathing

As a shark swims it takes in mouthfuls of water. The water is pumped over the gills and leaves the body through gill slits at each side of the head. Inside the gills there are feathery fringes which trap the oxygen from the water before it leaves the shark's body. In some kinds of shark, when it is not moving a body part called a spiracle is used to pump water over the gills. The spiracles are located just behind the eyes. The spiracle is either not there or poorly developed in fast swimming sharks.

Teeth

Sharks grow new teeth continuously as they break off. Some sharks can lose 30,000 teeth in their lifetime. New teeth grow in a groove inside the mouth and are replaced in rows. Lower teeth are mainly for holding prey while the upper teeth cut it.

Body differences between sharks and other fish

Sharks do not have a bony skeleton like other fish. Their skeletons are made of cartilage, which is more flexible than bone. Fish have a swim bladder inside their bodies that help their flotation in the water, and let them use less energy as they go up and down in the water. Sharks do not have a swim bladder.

Other differences between sharks and other fish are:

Sharks

Other fish

Have a cartilage skeleton

Have a bone & cartilage skeleton

Can only swim forwards

Can swim forwards & backwards

Have a large, oily liver

Have a gas-filled swim bladder

Have gill slits with no gill cover

Gill slits are covered

Eggs are fertilised in the female's body

Eggs usually fertilised in the water, outside the body

Scales are rough, like sandpaper

Scales are slippery & overlap

Upper part of tail fin is longer than lower part

Upper & lower tail fins are same size

Diet and hunting

Sharks eat almost anything, including fish, crustaceans, molluscs, marine mammals and other sharks. They attack from beneath and behind, bite the victim and swim a short distance away to wait for the victim to bleed to death. Great white sharks rely on stealth and surprise to catch prey such as seals. Whale sharks eat krill and other tiny creatures. Sharks can sense the electric impulses given off by living things, and locate their prey in this way.

Life cycle of sharks

A shark developing in an egg case. Getty Images

Sharks mate when the male deposits sperm into the female's body. Male sharks have claspers as part of their pelvic, or second pectoral, fins. These are placed into an opening in the female's body and sperm passes along grooves in the clasper.

There are three ways in which different sharks produce young:

Oviparous

Ovoviviparous

Viviparous

A shell or case is formed around the egg which protects it while it is developing. The female deposits the egg cases in the sea. Different
sharks have differently shaped egg cases.

Horn and swell sharks develop this way.

A thin tissue covers an egg or group of eggs (called a candle) and this stays inside the mother's body. After a while, the tissue is shed
and the young sharks continue to develop inside the mother's uterus (womb).

Mako sharks develop this way.

The young develop inside the mother's uterus and are born live. In some kinds of shark, the young eat each other before birth and the
strongest one is the only one to be born.

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